The prior art describes several alternatives to produce gasoline and distillate from synthesis gas that do not anticipate the present invention of four reaction stages with an overall recycle loop to produce commercial quality fuels. Chang et al (U.S. Pat. No. 3,894,102) and Zahner et al (U.S. Pat. No. 4,011,275) propose that synthesis gas be passed over methanol producing catalyst with an acid component activity to convert methanol to dimethylether and then feeding this intermediate mixed product to a fuel producing stage with recycle of light components to mix with the intermediate mixed product feed.
In another example, Chang et al (U.S. Pat. No. 4,076,761) use synthesis gas produced from coal, shale and/or residua that is conveyed to a carbon oxide converter and thence to a fuel producing stage with recycle of light gases back to the synthesis gas stage, the carbon oxide conversion stage or the fuel producing stage.
Garwood et al (U.S. Pat. No. 4,304,951) disclose the advantage of hydrotreating only the heavy fraction of product from a fuel producing stage using ZSM-5 catalyst. The hydrotreating step is carried out using essentially pure hydrogen and isolated from the prior three stages to produce liquid fuel from synthesis gas.
Thereby, the referenced patents proceed with four sequential stages with separation of liquid intermediates and product concentration steps after the first, third and fourth stages, resulting in a complex and low efficiency process. In addition, due to the production of high melting point (˜79° C.) durene, the cooling condenser ahead of the separator after the ZSM-5 stage needs a light gasoline recycle wash to keep it clean from durene deposition.
Haldor Topsoe (J. Topp-Jorgensen, “Topsoe Integrated Gasoline Synthesis—the TIGAS Process”, in D. M. Bibby, C. D. Chang, R. W. Howe, S. Yurchak (Eds.), Methane Conversion, 1988, Elsevier Science Publishers, B. V., Amsterdam, 293-305) simplified the Mobil Methanol-to-gasoline (MTG) scheme by combining the first three stages within one synthesis gas recycle loop without intermediate separation utilizing a proprietary catalyst for the first step to enable it to operate effectively at the lower pressures required by the ZSM-5 step. Methanol production is equilibrium limited and conversion would be enhanced by operation at high pressure. However, at high pressures, ZSM-5 produces increasing amounts of the undesirable component, durene. The proprietary catalyst produced DME in addition to methanol to increase the conversion to oxygenates. At elevated pressure, however, ZSM-5 produces a gasoline with a very high heavy aromatic content, in particular with high concentrations of durene that then would require hydrotreating as in the MTG New Zealand plant. Operating at about 20 atmospheres, the durene level was more than about three times a satisfactory level and it was stated, though not shown, that an isomerization step could be introduced into the loop to bring the durene content close to equilibrium, which would give a satisfactory product (FIG. 9 of the article). The article does not show that it was demonstrated. The olefinic content of the product was reduced as the pressure of hydrogen was increased and was overall lower than in the Mobil MTG product thereby producing lowered Research and Motor Octanes.
In Skov et al (U.S. Pat. No. 4,520,216), three stages are sequenced with no intermediate separation using a single recycle loop with interstage heat exchange. This scheme produces an undesirable high durene content fuel. Jorn et al (U.S. Pat. No. 4,481,305) proposes a very complex set of recycles for a three-reaction stage configuration.
In still another configuration, catalytic activities of the first three stages were integrated into one catalyst for a one stage process (F. Simard, U. A. Sedran, J. Sepulveda, N. S. Figoli, H. I. de Lasa, Applied Catalysis A: General 125 (1995):81-98). The one-stage conversion process used a combined synthesis gas/methanol and methanol-to-gasoline catalyst, a ZnO—Cr2O3+ZSM-5 catalyst, that produced gasoline compounds from synthesis gas feed, however, the selectivity to carbon dioxide was extremely high, ca 70%, making the process impractical. The overall reaction is described by 2nCO+nH2→(CH2)n+nCO2 with a minor amount of water (Javier Erena et al, Chemical Engineering Science 55 (2000) 1845-1855).
The complexity of the demonstrated and commercialized fixed bed Mobil Methanol-to-Gasoline (MTG) process can be appreciated from the description of the commercialized MTG process by Yurchak in D. M. Bibby, C. D. Chang, R. W. Howe, S. Yurchak (Eds.), Methane Conversion, 1988, Elsevier Science Publishers, B. V., Amsterdam, 251-272. In this process, synthesis gas is first converted to a methanol/water (CH3OH/H2O) mixture in a stand-alone plant. The methanol/water mixture is recovered and sent to intermediate tankage. Recycle is used to provide a heat sink for the highly exothermic reaction and to enhance synthesis gas conversion for this equilibrium limited reaction. The recycle gases are cooled to remove the methanol/water produced and must be reheated before returning to the reactor. The product methanol/water mixture from tankage is fed to a two stage reactor system containing a lead reactor with a catalyst that partially converts the methanol to dimethylether (DME) and then to another reactor with a recycle loop, the methanol-to-gasoline (MTG) reactor that converts the methanol/DME mixture to a heavy gasoline containing large amounts of durene, 1,2,4,5-tetramethyl benzene molecule that has a high freezing point (79.3° C.) and must be removed to make a viable gasoline product. The removal is effected by a hydrotreating step performed on a heavy fraction of the intermediate product from the fuel producing reactor stage and the hydrotreated fraction is combined with the light gasoline fraction to produce the gasoline product. The hydrotreater is operated at elevated pressure and is supplied with a hydrogen rich stream, which is produced from a portion of the synthesis gas by a separation step such as Pressure Swing Adsorption (PSA). The hydrotreating catalyst is presulfided and operated with a hydrogen rich gas recycle (Yurchak, 1985) and Garwood et al, (U.S. Pat. No. 4,304,951). One of the catalysts tested but rejected due to low activity is a presulfided commercial cobalt molybdate on alumina (CoMoOx/Al2O3) catalyst.
The commercial plant built and operated in New Zealand using this scheme has the complexity of three recycle catalyst loops and three separation steps involving cooling the intermediate products to liquefy them to enable conventional separation and distillation steps and stepping down of pressures and recompression, one for making methanol one for making the raw gasoline and the third for removing the durene. Typical catalysts and conditions used in each step in the Mobil MTG plant built in New Zealand are shown in Table 5 below. It is clear from this abbreviated description that this prior art process is quite complex and inefficient in its handling of intermediate products and the recycles and it requires several high cost high pressure feed and recycle compressors, and high pressure pumps.
TABLE 5(a)Prior Art MTG Reaction SequenceTypical ReactorTypical ReactorTemperature, C.Pressure, AtmPrincipal ReactionsFeedCatalystsNote (1)Note (2)CO + H2     CH3OH + H2OCO, H2Reduced230-29050-100CuO/ZnO/Al2O3CH3OH     (CH3)2O + H2OCH3OH, H2Oγ-Al2O3310-32018-22 pressureCH3OH     (CH2)n + n H2OCH3OH, (CH3)2O,ZSM-5350-36618-22n/2(CH3)2O     (CH2)n + n/2H2O H2ODurene     iso-Durene(CH2)n, H2Sulfided 220-27030-40Ni—W onSiO2/Al2O3/faujasiteNotes(1) (CH2)n with 4 < n < 10 denotes on the average the composition of the gasoline product which is a mixture of paraffins, iso-paraffins, olefins, cyclics and methyl substituted aromatics.Note(2) Reactor conditions from K.G. Allum and A.R. Williams, “Operation of the World's First Gas-to-Gasoline Plant”, in D.M. Bibby, et al (Editors), Methane Conversion, 1988, Elsevier Science Publishers, B.V., Amsterdam, p691-711.
In the Mobil MTG process, durene is produced in enough quantities to result in undesirable cold temperature performance of the gasoline and must consequently be reduced. It is shown in Sergei et al. (“Process Aging Studies in the Conversion of Methanol to Gasoline in a Fixed Bed Reactor”, Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979) that ZSM-5 produces durene in much larger quantities than expected from equilibrium. This is shown in Table 5(b), which is an abstract of Table V of the reference.
TABLE 5(b)time on stream in cycle, h11645maximum temperature, ° F.745745779779tetramethylbenzenes, mol %equilexptlequilexptldurene (1 ,2, 4, 5-)33.067.632.897.3isodurene (1, 2, 3, 5-)50.424.050.31.1prehnitine (1, 2, 3, 4-)16.68.416.91.6
Halving the amount of durene during an average cycle has been shown to produce a satisfactory fuel, therefore isomerizing the tetra-methyl-benzenes to an equilibrium mixture would be satisfactory to eliminate part of the problem. However, a certain amount of dealkylation of tetra-methyl-benzene is also provided by the catalyst used in the New Zealand plant (Garwood et al.).
Therefore, there remains a need for an efficient process to produce fuel from synthesis gas, whereby the fuel contains low amounts of durene and highly substituted benzenes for better viscometric properties in cold temperature performance.